The present invention relates generally to a method for transmitting real-time data packets in a cyclic communication system having transmission cycles that can be used for both real-time communication and non-real-time communication. More particularly, the invention relates to a corresponding communication system and a user of such a communication system.
A synchronous, clocked communication system with equidistance properties is a system that consists of at least two users interconnected via a data network for the purpose of mutually exchanging data or mutually transmitting data. The data exchange occurs cyclically in equidistant communication cycles that are predefined by the communication clock used in the system.
Users are, for example; central automation devices; programming, configuration or operating devices; peripheral devices, such as, input/output modules, drives, actuators, sensors, stored program controllers (SPCs) or other control units; and computers or similar machines that exchange electronic data with other machines, in particular, process data of other machines. Users are also referred to as network nodes or simply nodes.
The term control units as used hereinafter means closed loop or open loop control units of any kind, for example, coupling units (so-called switches) and/or switch controllers. The data networks used are, for example, bus systems, e.g., field bus, Profibus, Ethernet, Industrial Ethernet, FireWire or PC-internal bus systems (PCIs), etc., in particular, Isochronous Realtime Ethernet.
Data networks enable communication among a plurality of users through networking, i.e., the connection of the individual users among each other. Communication means the transmission of data between users. Data to be transmitted is sent as data messages, i.e., data is bundled into a plurality of packets and in this form is sent over the data network to the corresponding recipient. Such packets are known as data packets. The term data transmission as used in this document is completely synonymous with the aforementioned transmission of data messages or data packets.
U.S. Pat. No. 6,055,242 ('242) discloses a method and a device using a Broadband Link Protocol (BLP), which supports the use of a plurality of interactive and non-interactive digital services by means of HFC networks. In accordance with the method disclosed in the '242 patent, synchronous transfer mode (STM) information, variable length (VL) information and asynchronous transfer mode (ATM) cells can be transmitted.
In distributed automation systems, e.g., in the field of drive technology, specific data must arrive at specific times at the destined users and be processed by the recipients. This type of data transfer is often referred to as real-time critical data or data traffic because the failure of the data to arrive on-time at the destination leads to undesirable results with respect to the user. This contrasts with non-real time critical, e.g., Internet-based or intranet-based, data communication. In accordance with IEC 61491, EN61491 SERCOS Interface—Brief Technical Description, which can be viewed, for example, at the following website: http://www.sercos.de/pdf/sercos_kurzbeschreibung_de—0202.pdf, successful real-time critical data traffic of the aforementioned type can be guaranteed in distributed automation systems.
Today, automation components, (e.g., controls, drives, etc.) generally have an interface to a cyclically clocked communication system. An operation level of the automation component known as fast cycle, (e.g., position control in a control unit, torque control of a drive) is synchronized to the communication cycle. This determines the communication clock. Other, low-performance algorithms, known as slow cycle (e.g., temperature controls) of the automation system can likewise only communicate with other components (e.g., binary switches for fans, pumps, etc.) via this communication clock, although a slower cycle would be sufficient. Using only a single communication clock for transmitting all the information in the system places high demands on the bandwidth of the transmission path.
A peripheral image is composed of a sum of data sets exchanged with other automation devices through real-time communication. Data sets received by an automation device through real-time communication are input data. Data sets sent by an automation device through real-time communication are output data. In an automation device, input data are processed in a cyclically-called application program and output data are generated. The application program can be composed of a plurality of different functions that work with different data sets at different times. It is not necessary to call all the functions of the application program in each application cycle. As a result, not all input data is processed and new output data is generated in each application cycle.
Until now, in cyclic real-time communication, all the data of a peripheral image have been exchanged in each communication cycle, regardless of whether the input data were processed or new output data generated in each application cycle.